Process for molten salt bath electrolytic descaling of ferrous metal strip

ABSTRACT

Molten salt bath electrolytic descaling apparatus and process employing an electrical shield of aluminum around an immersed cast iron roll and electrically charged oppositely from the electrical charge on the strip to prevent current flow from the strip to the electrolyte through the roll.

United States Patent [111 3,617,455

[72] lnventors Robert H. Shoemaker [50] Field of Search 204/ l 45. 1, Royal Oak; 206 CD John A. Faler, Livonia, both of Mich.

[21] APPL 79 722 [56] References Cited [22] Filed Feb. 5, 1969 UNITED STATES PATENTS 1 Palmed Nov-1, 1971 2,468,006 4/1949 Webster 204/145 1 g Kolene Corporation 2,826,539 3/1953 Murtland, Jr 204/145 Detroit, Mich- 2,936,278 5/1960 Shoemaker et al. 204/243 x Primary Examiner- Howard S. Williams 5 PROCESS FOR MOLTEN SALT BATH ASSI'JIGIII Examiner-Neil A. Kaplan ELECTROLYTIC DESCALING or FERROUS A'wmekcunen METAL STRIP 1 Chum 2 Drawing Figs ABSTRACT: Molten salt bath electrolytic descaling apparatus [52] US. Cl 204/145, and process employing an electrical shield of aluminum 204/206 around an immersed cast iron roll and electrically charged op- [51] Int. Cl C23b 1/06, positely from the electrical charge on the strip to prevent cur- C23b 5/68 rent flow from the strip to the electrolyte through the roll.

PATENTED NI1V2 IBYI 3511455 LFIG.

INVENTORS ATTORNEYS PROCESS FOR MOLTEN SALT BATH ELECTROLYTIC DESCALING OF FERROUS METAL STRIP GENERAL DESCRIPTION It is known to descale ferrous metal strip continuously U.S. Pat. No. 2,468,006 of Apr. [9, 1949 by apparatus including a ferrous metal tank containing molten salt electrolyte; with a ferrous metal roll partly immersed in said electrolyte and mounted at its ends in the tank; with means for directing the strip to be descaled first into said electrolyte and then over an unimmersed or exposed part of said 'roll and thus out of said electrolyte and said tank; with means in the electrolyte near the roll for electrically charging the strip as it approaches the roll.

Such process as described in said U.S. Pat. No. 2,468,006 has long been known to the art and is often referred to as Kolene No. 4 Process. identified by originator Kolene Corporation of Detroit, Michigan.

In the past, such process has often resulted in arcing and burning of the strip being descaled during the period of current flow from the charging grid which electrically charges the strip as it approaches the roll and it has been discovered by the invention hereof that such harmful results are often due to the suspected flow of current from the charging grid to the cast iron roll commonly employed through the strip itself and with the circuit completed and thus current enabled to flow by the contact between the immersed part of the roll and the electrolyte as well as by the electrical conductivity established by the conventional mounting of the ends of the roll in the sides of the tank in a conductive manner.

OBJECTS OF THE INVENTION An object of the invention hereof therefore is to prevent such harmful results and this has been accomplished by employing an electrical shield around the cast iron roll. In the preferred embodiment here shown, such shield is of aluminum and is filled with molten salt and surrounds the immersed part of the roll and has an open top exposing the upper part of the roll over which the strip is directed. Such shield is charged oppositely from the charge placed on the strip and consequently there is built up and maintained on the surface of the shield an inert film which prevents current flow from the strip-charging grid through the cast iron roll, the circuit being disestablished by the isolation of such roll from the electrolyte in the tank and from the insulating mounting of the roll in the tank as here disclosed.

THE DISCLOSED EMBODIMENT The embodiment just described is disclosed in the appended drawing as a preferred embodiment.

In this drawing:

FIG. 1 is a diagrammatic view of a conventional apparatus equipped with the aluminum shield and other parts of the invention.

FIG. 2 is a fragmentary elevation view of the aluminum shield and associated cast iron roll only of the entire apparatus.

DETAILED DESCRIPTION The drawing shows apparatus for descaling a strip 20 which includes a ferrous metal tank 22 containing electrolyte of molten salt of the formulation of said U.S. Pat. No. 2,468,006, the electrolyte filling the tank up to the level 24. Suitably mounted on standards 26 are directing rolls 28 including particularly a large cast iron directing roll 32 which direct the strip 20 to be descaled first into the electrolyte and then over the unimmersed part of the roll 32 and thus out of the electrolyte and the tank 22.

The apparatus here disclosed in effect comprises two separate baths contained within the same tank 22, the bath in the leading or left-hand side of the tank being isolated by an electrical barrier 34 from the bath at the trailing or right side of the tank 22.

In each bath is a means 36-38 for electrically charging the strip as it approaches directing rolls, with such means being within the electrolyte and near the adjacent roll. The left-hand bath is negative and the means 36 comprises a negative power supply grid of conventional form negatively charged from any external negative charging apparatus as illustrated diagrammatically. The right-hand bath is a positive bath and thus the grid 38 is positively charged as illustrated.

THE IMPROVEMENT What has been described thus far is conventional. What is now to be described is the improvement of the invention.

First the rolls 32 are insulatedly mounted in the tank 22 to prevent current fiow from the rolls to the tank.

Secondly, an aluminum shield 40 is built around the rolls 32 isolating the roll from the electrolyte.

The shield 40 used is in the form of an open top tub of aluminum and is located under and around the immersed surface of the roll 32 and is adjacent to said roll but is spaced from it and is filled with the molten salt.

Third, means are provided and are shown diagrammatically as an electrical lead 48 for electrically charging the shield 40 in a manner to prevent current flow from the strip-charging means 36 to the electrolyte through the strip 20 and the roll 32.

Fourth, the strip-charging means 36 charges the strip oppositely from the manner in which the shield charging means 48 charges the shield. Thus in the left-hand tank wherein the strip-charging means 38 charges the strip cathodically or negatively, the shield-charging means 38 charges the strip cathodically or negatively, the shield-charging means 48 charges the shield anodically or positively.

PROCESS The invention that has been described in terms of apparatus in the foregoing may equally be described in terms of process as follows.

Starting with the conventional process wherein a ferrous metal strip is being descaled electrolytically by a molten salt bath in a ferrous metal tank, and wherein the strip is directed over a ferrous metal roll partly immersed in the electrolyte and mounted at its ends in the tank and wherein the strip is electrically charged as it approaches the roll, the improvement in the process is as follows.

First, the roll over which the strip is directed, namely roll 32 is maintained electrically insulated from the tank 22.

Secondly, the immersed part of the roll 32 is shielded and isolated from the electrolyte by the use of an aluminum shield 40 around the roll 32.

Third, the shield 40 is charged electrically in a manner to prevent current flow from the strip-charging means 36 to the electrolyte through the strip 20 and the roll 32.

Fourth, the charge on the strip from means 36 is opposite from the charge on the shield from means 48.

OTHER OBSERVATIONS It is of course understood that the shield is continuously charged during the electrolytic operation for the purposes desired. HOwever, once the film is built up on the shield it is established and from then on is merely maintained during the electrolytic operation.

It is important to note that current flow from the strip to the roll 32 is disestablished by the fact that the roll 32 is electrically insulated from the tank 22 as well as being electrically isolated from the electrolyte by the shield.

While in the illustration here given but one shield 40 is shown, and this shield is anodically charged and is located around the roll 32 of the cathodic bath, the expedient here illustrated as new may be employed in other parts of the bath and process, as for example, around roll 32 of the positive bath. Of course, it will be required that the means for charging the shield operates to charge the shield oppositely from the means in the same bath which charges the bath itself by charging the strip.

lt is believed that the charging of the salt-filled aluminum shield builds up a nonconducting film of aluminum oxide on the shield; such film forms an insulating barrier between the roll 32 and the electrolyte.

CONCLUSION and then over an unimmersed part of said roll and thus out of said electrolyte and said tank; and wherein the strip, in said electrolyte, near said roll, is electrically charged as it approaches said roll;

the improvement which comprises a. maintaining the roll electrically insulated from the tank;

b. shielding and isolating the immersed part of said roll from the electrolyte by an aluminum shield around the roll;

c. charging said shield in a manner to prevent current flow from the strip-charging means to the electrolyte through the strip and the roll;

d. with the charge on the strip being opposite from the charge on the shield.

i i i i 

